Digitally modulated signals are used to transport high-speed data, video and voice on cable networks. The high-speed signals may be subject to a variety of impairments that can seriously impact the quality and reliability of the services being provided. One such potential impairment is known as Hum, and is caused by an ingress of the AC power line signal onto a coax cable plant. In the US, Hum appears as variations of the power of a TV signal at 60 Hz and/or its second harmonic 120 Hz, while in Europe corresponding frequencies are 50 Hz and 100 Hz.
The degree to which Hum is present in a TV signal is a key indicator of the “health” of a cable TV network, and is a required measurement per FCC regulation. Traditionally, the presence of Hum has been assessed by measuring a level fluctuation of sync tips of an analog TV signal, serving as reference points in time. However, in recent years the Community Antenna TeleVision (CATV) network has evolved from an unidirectional analog system to a bi-directional, Hybrid Fiber Coaxial with a mix of analog and digital signals, and it is expected that the digital portion of the CATV network will continue to increase. As the CATV industry is evolving towards total digital, in the future there may not be any analog TV signals left in the channel line-up to enable Hum measurement. Since there is no amplitude references in the digital TV signal, there is a need to find a different way to measure Hum that would be suitable for assessing the quality of digital TV signals.
Delivery of data services over cable television systems is typically compliant with a data-over-cable-service-interface-specifications (DOCSIS) standard. Digital TV (DTV) signals propagating in the coax cable plant of the CATV network typically include signals that are modulated using quadrature amplitude modulation (QAM), and can be referred to as QAM digital signals or QAM digital channel signals. In the United States, 64-QAM and 256-QAM are the mandated modulation schemes for digital cable as standardised by the SCTE in the standard ANSI/SCTE 07 2000.
There currently exists or have been disclosed a variety of test instruments for measuring different parameters representing the quality of a digital QAM TV signal in the CATV coax cable plant. To troubleshoot a subscriber's premises with a signal problem, a technician will travel to the premises or a hub nearby, and conduct a variety of tests on the digitally modulated signal, e.g. RF level, MER, pre- and post-FEC BER, and an evaluation of the constellation for impairments. In addition, the technician may look at the equalizer graph for evidence of micro-reflections, and check in-channel frequency response and group delay. Moreover, if the QAM analyzer is able, the measurements are repeated in the upstream direction. Hand-held battery-powered testing devices for testing DTV signals are a particularly convenient tool for performing TV signal measurements in the field. Unfortunately, such devices currently do not have the capability of measuring Hum on DTV signals.
U.S. Pat. No. 7,142,609, which is incorporated herein by reference, discloses a test instrument, which teaches a method and an apparatus for detecting and quantifying impairments in digital QAM signals. This patent teaches measuring Hum by de-modulating the digital QAM signal, and by using a spectrum analyzer functionality to sample the I and Q quadrature components of the de-modulated QAM signal and to trigger the sampling sequences to the power line to extract Hum-related modulation of the TV signal.
One drawback of this approach is that it requires the use of a power line for triggering, which is not available in a battery operated device.
Another drawback of this approach is that Hum components that are not synchronized with the 60 Hz line rate, for example disturbances in a TV signal that are related to faults in switching-type power supply circuits that are often used in trunk amplifiers and line extenders in a CATV network, may not be properly evaluated. Furthermore, using a power line for triggering in measuring Hum may fail if Hum is related to a battery back-up power supply which is not synchronized with the main power supply.
Accordingly, there is a need to have a simple and efficient method for measuring Hum on a digital QAM carrier with a battery-operated tester device for testing QAM digital signals.